1. Field of the Invention
The present invention relates to a battery voltage measurement apparatus for detecting battery voltage of power supply unit including a plurality of secondary batteries connected in series, and more particularly to a battery voltage measurement apparatus for immediately detecting a disconnection of a signal line when a signal line for measuring voltage is disconnected.
2. Description of Related Art
A hybrid vehicle having a motor along with an engine as a driving source for running a vehicle has been widely known. One kind of control (mode) for the hybrid vehicles includes a parallel hybrid vehicle whose motor helps boost engine power. This parallel hybrid vehicle attempts to satisfy a driver's demand by performing various controls while securing remaining amount of a battery. The controls are for example assisting engine power using a motor at acceleration and charging a battery and such. The battery is usually comprised of a plurality of battery cells connected in series because high voltage is required.
A plurality of voltage sensors are used for detecting a voltage of this battery. A plurality of battery cells connected in series to constitute a battery is divided into an appropriate number of cells and modularized. The plurality of voltage sensors are provided to each module and connected with each module by voltage measurement lines.
When a disconnection occurred on a voltage measurement line, it is important to immediately detect the disconnection and take a failsafe action. Accordingly, the technique disclosed in Japanese Unexamined Patent Publication No. 2001-116776 describes that in general, each module is connected with a resistor for detecting measurement line disconnection in addition to a voltage sensor comprised of semiconductor integrated circuits and filter portion. FIG. 4 is a view showing a voltage measurement apparatus described in this technique. In the voltage measurement apparatus illustrated in FIG. 4, resistors for detecting disconnection 2-1 to 2-n are placed. Resistance values of resistors for detecting disconnection 2-1 to 2-n are alternately placed in each module with different values. In the apparatus illustrated in FIG. 4, if a voltage measurement line Q is disconnected for example, a voltage value detected by voltage sensors 4-1 and 4-2 will be the value corresponding to a resistance ratio of the resistors for detecting measurement line disconnection 2-1 and 2-2 respectively. This allows the voltage sensors 4-1 and 4-2 to detect a voltage not normally detected, consequently the disconnection of the voltage measurement line can be immediately detected.
However it has now been discovered that a voltage measurement apparatus of a conventional technique requires that resistances for detecting measurement line disconnection 2-1 to 2-n to be inserted in parallel to each module, where the resistances need to be several MΩ so as to suppress current in standby mode (dark current). Nevertheless a detectable resistance value is limited, and thus it is difficult to reduce current below a resistance value being limited.